Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. Many scientific and communications missions, however, cannot be accurately fulfilled without consistently monitoring and controlling the 3-axis attitude of the spacecraft. In many applications, the satellite must be positioned to direct communication signals in particular directions or to receive signals from specifically located sources. Without accurate control over spacecraft 3-axis attitude, the transmission of such signals is hindered and at times impossible.
Many modem spacecraft use star trackers for 3-axis attitude control. The systems for 3-axis attitude determination generally include one or more star trackers and a separate 3-axis orthogonal gyroscope. During normal operation, star trackers provide continuous attitude information and the 3-axis orthogonal gyroscope is needed to provide rate information and is constantly corrected by the star trackers.
To improve spacecraft attitude control performance, stellar inertial attitude determination algorithms employ a carefully designed star catalog. The current known art in star selection for star trackers mainly concentrates on generating star catalogs with certain properties. Typically, a star catalog is "equalized" by eliminating stars in the dense areas of the star catalog. The resulting star catalog becomes approximately uniformly distributed and is known to improve stellar inertial attitude determination performance.
Typically, computer memory and algorithm complexities (computer throughput) have always limited the size of on-board star catalog and prevented star catalog designs from using real-time star selection from an on-board star catalog for attitude determination purposes. Unfortunately, real-time star selection algorithms are complicated and have difficulty accommodating other star selection criteria.
The disadvantages associated with these conventional star catalog techniques have made it apparent that a new technique for star catalog equalization to enhance satellite attitude determination is needed. Preferably, the new technique would allow real time star selection while allowing the inclusion of other star selection techniques. Additionally, the new technique should improve stellar inertial attitude determination performance. The present invention is directed to these ends.